Waste water treatment, media therefor and its manufacture

ABSTRACT

Particulate material for use in waste water treatment. The material comprises granules of a plastic material (e.g. poly-ethylene), each of which contains a plurality of grains of a substantially inert mineral (e.g. sand) coated thereon to provide a habitat for microorganisms effective in waste water treatment. The granules will have a predetermined particle size which is governed by the particle size range of the granules, e.g 3-10 mm and the grains will have a predetermined particle size range, e.g. 0.1-3.5 mm.

TECHNICAL FIELD

This invention relates to loose particulate material for use in wastewater treatment plant and methods, to the manufacture of such material,and to methods and apparatus for the treatment of waste water using suchmaterial.

BACKGROUND ART

Waste water may be treated by gasification, for example by the aerationor oxygenation of sewage or other waste water containing organic matterdegradable by the action of oxygen thereon. A wide range of treatmentmethods and apparatus has been used and proposed. Oxygen does notdissolve easily or quickly in water and it is therefore in principledesirable to utilize fine bubble aerators wherein the bubbles are lessthan 2 mm and desirably less than 1 mm in diameter. Smaller bubbles havea larger specific surface area for oxygen transfer into the liquid, andalso rise more slowly through the liquid to give a longer time for theoxygen to transfer before the bubble reaches the liquids surface.Treatment plants are known comprising a treatment vessel with aeratordevices submerged in the waste water to produce the bubbles.

It has also been proposed to provide a treatment plant where thetreatment vessel contains a bed of loose material. Aeration then causesa degree of fluidization of the bed and sustains the growth of apopulation of microorganisms on the material of the bed. In the presenceof dissolved oxygen the microorganisms convert the organic matter in thewaste water to carbon dioxide, water and to more bulky cellularmaterials and sludge thus alleviating the biological oxygen demand(BOD). Under appropriate operating conditions they will also convertammonia to nitrate compounds. The surplus sludge thus formed can passout with the effluent for eventual separation and recycling if desired.

Problems of fouling and clogging of the aerator devices and any pipework can be acute where they are buried or caged beneath a bed of loosematerial. Regular closing and draining of the plant for cleaning andunblocking or replacement of the aerators is inefficient and expensivedue to the need also to move aside or remove the filter bed material togain access to the buried aerators.

Our Application no. PCT/GB94/02795 (now publication WO/95/17351)describes a method and apparatus for treatment of waste water whereinsuch problems are mitigated, and in particular discloses looseparticulate material for use as a fluidizable bed in the waste watertreatment, said material being characterized by particles of asubstantially inert mineral adhered to, coated on or coated by plasticsmaterial to provide a habitat for microorganisms effective in wastewater treatment.

That Application also discloses a waste water treatment methodcharacterized by allowing waste water to enter a treatment vesselcontaining a bed of such loose particulate material, and gasifying thebed and the waste water by means of gas bubbles emitted from one or moregasifiers disposed within the bed and adapted for placement and removalfrom above the water level.

That Application also discloses apparatus for treatment of waste watercharacterized by a treatment vessel containing a bed of such looseparticulate material, one or more gasifiers movably disposed within thebed, and means for supplying gas to the gasifiers for emission as gasbubbles to gasify the bed and the waste water, said gasifiers beingadapted for placement and removal from above the water level.

That Application discloses such loose particulate material having adensity in the range of from substantially 1.0 to substantially 1.3g/cc, having a specific surface area in excess of approximately 600 m²per cubic metre of the loose material, and having a particle size rangeof substantially 3 mm to substantially 10 mm in diameter. An example ofthe material is disclosed as particles of sand or gravel or other inertmineral particles at least partially adhered to, coated on or coated byplastics material, preferably a thermoplastics material such aspolyethylene. It was disclosed that the material could be produced to adesired density for a particular application by changing the initialproportions of mineral and plastics. The particles may be formed bypartially melting the polyethylene, for example in hot air, and allowingit to contact the sand or gravel. In use, the particles are quite looseand readily permit the gasifiers to be shaken down through the bed andplaced where desired on the base of the vessel. No surrounding cage,grid or mesh was required.

Such loose particulate material has been found in practice to beeffective for use as a fluidizable bed in waste water treatment by suchmethods and apparatus. Such materials have been found to provide aparticularly suitable habitat for a high population density ofmicroorganisms of the type effective in waste water treatment.

DISCLOSURE OF INVENTION

It has been found, however, that particularly effective and efficienttreatment can be obtained for a given plant and treatment requirementsif the physical characteristics of the loose particulate material arespecified and controlled to suit the requirements. An object of thepresent invention is to provide such loose particulate material and toprovide methods for its manufacture.

According to the present invention there is provided a loose particulatematerial for use in waste water treatment, said material characterizedby granules of plastics material each having a plurality of grains of asubstantially inert mineral coated thereon to provide a habitat formicroorganisms effective in waste water treatment, said granules havinga predetermined particle size range, and said grains having apredetermined particle size range and being disposed at a predeterminedpacking density range on the granules.

The loose particulate material may be regarded as a fluidizable bed.Substantial trials of such fluidizable beds in practice have howevershown that certain problems can arise probably as a natural consequenceof the achieved higher population density and therefore compactness ofthe plant for a given incoming waste water load.

Thus, according to a further aspect of the present invention there isprovided loose particulate material for use in waste water treatment,said material being characterized by granules of plastics material eachhaving a plurality of grains of a substantially inert mineral coatedthereon to provide a habitat for microorganisms effective in waste watertreatment, said granules having a predetermined particle size range,said grains having a predetermined particle size range and beingdisposed at a predetermined packing density range on the granules, andthe particles of said loose particulate material having an averagedensity of approximately 1.0 g/cc such that a proportion of theparticles tend to float and a proportion tend to sink in waste water tobe treated.

The material is preferably such that about 50% of the particles tend tofloat at the surface of a body of waste water to be treated and about50% tend to sink upon initial charging and when the body of waste wateris at rest. Upon commencing aeration, it is found that the particles mixvery rapidly into a vigorous circulation pattern with minimum energyinput requirements and highly efficient aeration.

The invention also provides waste water treatment plants and methodsusing such loose particulate materials.

The invention further provides a method of manufacture of looseparticulate material for use in waste water treatment, said methodcomprising contacting the granules of plastics material of apredetermined particle size range with a mixture of grains of asubstantially inert mineral of a predetermined particle size range andgrains of a soluble substance of a predetermined particle size range, atan elevated temperature, to coat the granules with the mixture, andsubsequently dissolving the soluble substance grains from the coating toprovide granules coated with said grains of substantially inert mineralin a predetermined packing density range.

It has been found that efficient and effective waste water treatmentrequires loose particulate material of a predetermined particle sizerange, according to the treatment requirements for that waste water.This is governed by the particle size range of the plastics materialgranules, typically 3 to 10 mm considered as a sieve size range andbearing in mind the generally irregular shape of such granules ascommercially available. The preferred size range is 4 to 8 mm, morepreferably about 4 to 6 mm. A suitable source of supply of said granulesis commercially available recycled chips of plastics material,preferably granulated from moulded rejects such as plastics boxes.

It is then generally desired to achieve a high specific surface area forthe loose particulate material, for example in excess of about 600 m²per cubic metre. The greater the specific surface area, thecorrespondingly larger is the habitat area for the microorganismseffective in the treatment. It would be appreciated that a larger numberof smaller grains of the mineral will provide a higher specific surfacearea, and hence it is generally desirable to select a fairly small grainsize for the mineral, for example from about 0.1 to 3.5 mm, preferably0.1 to 2.5 mm but again depending on the requirements for the specificwaste water treatment.

The specific gravity of the plastics material is fixed by the choice ofplastics material, suitably polyethylene, and is less than 1.0 g/cc.Likewise the specific gravity of the mineral is fixed by the choice ofmineral, suitably sharp sand, and is substantially greater than 1.0g/cc. Accordingly the resultant specific gravity of the looseparticulate material is inevitably determined by the initial choices ofparticle size for the plastics material and for the mineral. The surfacearea of the plastics material granule increases only as the square ofits diameter while its volume increases as the cube of its diameter. Thethickness and therefore effective volume of the mineral coating thereondepends on the diameter of the mineral grains over a given plasticsmaterial granule surface area.

Therefore an initial choice of particle size ranges for a given plasticsmaterial and given mineral constrains the specific gravity of theresultant particles to a predetermined value. That value may not bedesirable. In practice, if the specific gravity is substantially inexcess of 1.0 g/cc then the particles may in use remain undesirablycompacted as a bed at the base of the treatment vessel and not fullycirculate with the waste water circulation pattern induced by theaerator bubble stream. In this context it is assumed that the specificgravity of the waste water is normally close to 1.0 g/cc. If thespecific gravity of the particles is less than 1.0 g/cc then theparticles may in use remain undesirably concentrated in a floating massat the top of the treatment vessel and again not circulate as desired.

It is generally desirable for effective waste water treatment that thelarge majority of the fluidized bed particles circulate reasonablyvigorously in the waste water circulation pattern induced by theaeration. The achievement of the present invention is to permitprovision of loose particulate material having a specific gravity inaccordance with a desired predetermined value, and depending onparameters of the waste water being treated and the treatment plantitself, particularly its horizontal and water depth dimensions.

The invention thus permits the manufacture of the loose particulatematerial wherein it is reasonably possible to predetermine not only thedesired particle size ranges of the plastics material granules and themineral grains thereon, but also the specific gravity of the finishedmaterial. This is achieved by appreciating that it is possible tocontrol the effective packing density of the mineral grains in thecoating on the plastics material granules.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a cross-section through a particle of material having asubstantially uniform coating;

FIG. 2 is a cross-section through a particle having a controlled coatingpacking density; and

FIG. 3 is a cross-section through a particle having a differentcontrolled coating packing density.

MODES FOR CARRYING OUT THE INVENTION

FIG. 1 reproduces FIG. 3 of our above-mentioned WO/95/17351. It shows agranule 80 of polyethylene having a large plurality of grains 81 ofsharp sand adhered thereto as a coating by partial melting so aspartially to embed the grains on the granules.

FIG. 2 shows a similar granule 80 having a coating of grains of sharpsand of controlled reduced packing density, so as to provide a particleof predetermined desired specific gravity, and FIG. 3 shows a similargranule 80 having a controlled and lesser coating packing density toprovide a particle of predetermined lesser desired specific gravity.

The particles of FIGS. 2 and 3 can be manufactured as follows. Theparticle size ranges for the plastics material granules and the sandgrains are selected, usually in accordance with the waste watertreatment process and plant requirements. The mineral grains, e.g. sharpsand, are intimately mixed in a chosen proportion, for example 50:50,with grains of a soluble substance also of a predetermined particle sizerange. The soluble substance may be common salt, i.e. sodium chloride.The particle size range of ordinary table salt is about 0.15 mm and ofordinary water softening salt about 3.3 mm.

The plastics material granules are heated until they partially melt,i.e. the surface becomes shiny and tacky. They are then allowed tocontact the mineral mixture, for example by being rolled in a tray ofthe mixture. Each granule is thereby coated with an adhered coating ofthe mixture. The coated granules are then washed with cold water,preferably while they are still warm. The soluble salt grains dissolveaway in the water leaving the granules coated substantially solely withthe grains of sharp sand. The wash water can then be treated to removethe dissolved salt and may be disposed of for another use or may berecycled.

The soluble substance may be any economically available granularsubstance which is soluble in a common solvent, such as water, that doesnot affect the plastics material or the sand, and provided that it has amelting point higher than the softening point of the polyethylenegranules, i.e. higher than about 200° C.

The packing density can be controlled by selection of the particle sizerange of the soluble substance grains in relation to the particle sizerange of the sand grains. FIG. 2 shows the resultant particle when thesize ranges are substantially identical, e.g. each about 0.25 mm on an 8mm plastics material granule. FIG. 3 shows the resultant particle whenthe salt grains were about three times the diameter of the sand grains,e.g. 0.5 mm salt grains and 0.15 mm sand grains.

In a preferred embodiment, the packing density is controlled by varyingthe initial proportions of sand to salt grains in the mixture, and itwill be appreciated that this has a similar effect to varying therespective particle size ranges. Thus FIG. 3 can equally be taken toshow the resultant particle when the initial mixture contained a ratioof about 1:3 of sand to salt grains of substantially the same grainsize. Both the initial proportions and the respective grain sizes can beindependently varied. An advantage of controlling the packing density byusing sand and salt grains of substantially the same grain size andvarying their initial proportions in the mixture, is that it simplifiesthe production of a thorough mixture of the sand and salt grains.

The invention thus permits the manufacture of particles of a desiredspecific gravity for a particular treatment method and plant, and alsoparticles with a desired specific surface area, e.g. in the range 300 m²per cubic metre to 600 m² per cubic metre, for a particular treatmentmethod and plant. This further enables the initial charging of a plantwith particles having graded properties. For example a fluidized bedmight be created as a layered composite with loose particulate materialof relatively higher density at the base and lower density at the topwith perhaps also an intermediate layer of intermediate density. In usethe denser particles would circulate slowly primarily in the depths ofthe vessel for initial coarse aeration and a degree of filtration, withthe lighter particles circulating more vigorously primarily in the upperlevels to complete aeration of the waste water. Slightly differentmicroorganism populations could develop at the different levels withbeneficial consequences for the water treatment.

In another embodiment the particles are prepared for a particular plantsuch that a proportion, for example about 50%, tend to float and aproportion, for example about 50%, tend to sink upon initial chargingand when the waste water is at rest.

The inventors have been surprised to find that the use of such light,i.e. low density, material has led to unexpected advantages. Not only isthe efficiency of the waste water treatment maintained at a high level,but this is now coupled with simplicity and ease of maintenance of theplant. It is believed that this may be due to breaking away from theprevious tendency in the art to consider the particulate material as afluidizable bed, with the consequent implication that the materialshould settle down as a bed on the base of the treatment vessel when theaerators are turned off, and hence the particles should have a densitygreater than 1.0 g/cc. This was at least partly because it was regardedas necessary for the bed particles to have a filtration function.

It is now found that the treatment method is improved by maintaining themedia particles continually mobile within and substantially throughoutthe body of waste water within the treatment vessel, i.e. the particlesshould have the above-mentioned average density of approximately 1.0g/cc such that a proportion of the particles tend to float and aproportion tend to sink in the waste water, with a number suspended inthe body. In consequence, during aeration by the gasifiers, theparticles readily become continually mobile throughout the vessel. Thisensures high efficiency of contact between the air, the microorganismsand their food particles in the waste water being treated. Moreover, themobile particles enhance oxygen transfer by impeding the progress of thebubbles up towards the surface. When a bubble impinges on a particle itwill either break up into smaller bubbles or at least slow down as itmoves around the rough surface of the particle.

The method is found to be highly stable in that a sufficient populationof microorganisms adhere to the suspended media particles at all times,neither too many nor too few being retained over a period of time as theinevitable widely varying quantities of waste water input are receivedby and flow through the treatment vessel. This achieves the longer termstability that plant operators require, particularly for unattendedplants, and leads to the above-mentioned simplicity and ease ofmaintenance.

Because the average density of the particles is effectively the same asthat of the waste water, i.e. they have neutral buoyancy, the energyrequirement for aeration and circulation is minimized. It will beappreciated that energization of the aerators not only achieves aerationof the waste water but also drives the circulation pattern within thetreatment vessel, as described in our published International PatentApplication No. WO 95/17351. The present media particles are welladapted for effective use in such treatment plant and methods asdescribed in that published Application.

The efficiency in treatment by the suspended and circulatedmicroorganisms on the particles, and the efficiency in energyutilization can lead to a very high load factor and thus a smaller plantfor a given load, but still coupled with stability of operation,simplicity and ease of maintenance.

Because of the efficiency, the treatment vessel can be charged as low as20% of its volume with the particulate material, as compared with 60 to80% by volume with certain bulky prior art treatment media.

The particles can be made by contacting the granules of plasticsmaterial of a predetermined particle size range with a mixture of grainsof a substantially inert mineral of a predetermined particle size rangeand grains of a soluble substance of a predetermined particle size range(suitably the same as that of the inert mineral), at an elevatedtemperature, to coat the granules with the mixture, and subsequentlydissolving the soluble substance grains from the coating to providegranules coated with said grains of substantially inert mineral in apredetermined packing density range.

The mineral is suitably silver sand, also known as washed sand orplaypit sand, and is characterized by having a high percentage of grainsof fairly uniform size.

With prior knowledge of the densities of the plastics material and ofthe inert mineral, it is therefore readily possible to select thepacking density range to achieve the desired average density ofapproximately 1.0 g/cc.

Moreover, the contact conditions between the granules and the mixture ofgrains, and the elevated contact temperature, can be adjusted such thatthe grains coat the granules by burying themselves significantly intothe partially melted outer surface of the granules. Thus the grains mayhave about 50% of their volume buried and the other 50% exposed proud ofthe surface, so as to retain them securely in position on the granulesduring their subsequent use in waste water treatment.

The inventors have also found that, by using such significantly buriedgrains, the subsequent step of dissolving away the soluble substancegrains leaves concavities in the surface of the granules having quitecomparable overall surface areas to that of the adjacent convexities ofthe retained insoluble inert material grains. This gives the importanttechnical advantage that the specific surface area of the looseparticulate material particles is essentially independent of the initialproportions of numbers of grains of inert material and numbers of grainsof soluble substance.

Accordingly the initial proportions may be selected over a wide range,for example from 1:1 up to 8:1 or even higher of salt:sand, withoutcompromising the high specific surface area, for example in excess ofabout 600 m² per cubic metre as mentioned above.

It will be appreciated that a large number of smaller grains willprovide a higher specific surface area, but the advantage of usingsignificantly buried grains is that the grain size can be selected for aparticular specific surface area independent of selection of the ratioof salt:sand. The invention has thus achieved loose particulate materialoptimally adapted for efficient and effective waste water treatment.

We claim:
 1. A method for preparing particulate material for use inwaste water treatment comprising the steps of:a) contacting granules ofplastics material with a mixture of grains of a substantially inertmaterial and grains of a substance soluble in a solvent which does notaffect the plastics material or the substantially inert material at anelevated temperature, to coat the granules with the mixture; and b)subsequently contacting the coated granules with a solvent to dissolvethe substance soluble in said solvent from the coating to providegranules coated with said grains of the substantially inert material ina predetermined packing density range.
 2. A method according to claim 1characterized in that the grains of the substantially inert material andthe grains of the substance soluble in said solvent have substantiallythe same particle size range.
 3. A method according to claim 1characterized in that the ratio of the grains of the substantially inertmaterial to the grains of the substance soluble in said solvent is about1:8.
 4. A method according to claim 1 characterized in that saidgranules are contacted with said mixture at an elevated temperature suchthat the grains coat the granules by burying themselves into a partiallymelted outer surface of the granules, whereby step b) results in theformation of concavities in the granule surface.
 5. A method accordingto claim 1, wherein the grains of the substantially inert material andthe grains of the substance soluble in said solvent each have particlesizes in the range of 0.1 to 3.5 mm.
 6. A method according to claim 1,wherein the plastics material comprises polyethylene.
 7. A methodaccording to claim 1, wherein the substantially inert material comprisessand.
 8. A method according to claim 1, wherein the substance soluble insaid solvent comprises sodium chloride.
 9. A method according to claim1, wherein the solvent comprises water.